The present invention relates to a communication method and a communication system, and, more particularly to a communication method and a communication system used for a radio communication system that uses Data Rate Control (DRC) and Data Source Control (DSC).
In recent years, in a packet communication system and a mobile communication system, data communication such as transmission and reception of mails using mobile terminals, access to the Internet, and Web browsing has been actively performed. A system for such data communication using mobile terminals includes, for example, a communication terminal apparatus (Access Terminal, hereinafter referred to as AT) serving as a mobile terminal, a communication connection apparatus (Base Station, hereinafter referred to as BS) serving as a fixed station, a communication control apparatus (Base Station Control, hereinafter referred to as BSC) that connects plural communication connection apparatuses, and an external network such as a public network. When the BSC receives data from the external network, the BSC transfers the data to the BS. The BS transfers data to the AT according to radio communication. In order to realize such data communication, for example, it has become a general practice to follow the standardized data transmission procedure described in 3GPP2C.S0024 (Non-patent Document 1).
When the AT moves, the AT is required to switch a communication destination from a certain BS to another BS and continue the data communication. This switching operation is referred to as Handoff. As a basic operation of the Handoff, for example, there are operations such as securing of a radio communication path with a moving destination BS, switching of a data reception source BS, and opening of a radio communication path with a moving source BS.
In the securing and the opening of the radio communication path with the moving destination BS, the AT requests the BS and the BSC to secure and open the radio communication path. In the switching of the data reception source BS, the AT has a function of selecting a BS with a best reception state on the basis of reception intensity of a radio wave and notifying a result of the selection to the BS and the BSC as a Data Rate Control (DRC) signal. The BSC has a function of determining a BS at a transfer destination of data on the basis of a DRC received from the AT. The BS has a function of transmitting the data to the AT only when the BS receives a DRC, which is transferred to the BS, from the AT. The DRC signal includes a DRC Cover value. The DRC Cover value is determined for each BS. The AT notifies the BS of a DRC Cover value of a BS selected as a communication destination by carrying the DRC Cover value on a DRC signal. These methods of selecting a communication destination using the DRC are described in detail in, for example, Chapter 8.4 of C.S0024 (Non-patent Document 1).
When data is lost while the data is transferred from the BSC to the AT, in order to transmit the data again to compensate for the loss, the BSC has a function of accumulating data for retransmission, a function of granting continuous sequence numbers to the respective data at the time of data transmission, and a function of retransmitting the data in response to a retransmission request from the AT. The AT has a function of checking the sequence numbers granted to the data to detect loss of the data and a function of transmitting a message for requesting retransmission of the lost data (a Nak message) to the BSC. These methods of retransmitting data using the Nak message are described in detail in Chapter 3.4 of C.S0024 (Non-patent Document 1).
In a system of the Handoff, from the time when the AT carries out switching of a DRC according to a change in reception radio wave intensity until the time when the BSC changes a BS to be a transmission destination of data, the data arrives at the moving destination BS from the BSC, and the data is transmitted from the BS to the AT, a period in which the data does not flow on the radio communication path appears. When such a period appears, inconvenience occurs in an operation of an application in which it is expected that the data continuously arrive or a fall of throughput occurs. As a method of solving this problem, for example, 3GPP2 C.S0024-A (Non-patent Document 2) is disclosed.
In the Non-patent Document 2, a system for notifying a BS and a BSC of a BS at a Handoff destination from an AT using a DSC (Data Source Control) signal a fixed time before the AT issues an instruction for switching DRC is described. For this purpose, the AT has a function of notifying the BS and the BSC of the DSC switching instruction before switching the DRC. The BSC has a function of not only transmitting data to a BS indicated by a DRC but also transmitting data to a BS indicated by a DSC. Since the BSC receives the notice of the DSC in this way, the BSC can predict DRC switching in advance. By transmitting identical data to plural BSs from the BSC, the data is transmitted to the AT from the BS without spending time at the time of the DRC switching. This makes it possible to eliminate a no-communication period at the time of DRC switching, which is a problem in the Non-patent Document 1. These methods of transmitting data using the DSC are explained in detail in, for example, Chapter 11.7.6.1.5 and Chapter 14.2.1.3.3.3 of C.S0024-A (Non-patent Document 2).
In a method disclosed, when a receiver detects loss of segments, the receiver transmits a minimum sequence number of the lost segments and a confirmation response including a value indicating the number of the lost segments as an advertised window size to a transmitter (see, for example, Patent Document 1).    [Patent Document 1] JP-A-2004-343552    [Non-patent Document 1] 3GPP2 C.S0024 cdma2000 High Rate Packet Data Air Interface Specification Version 4.0, Chapter 3.4 and Chapter 8.4    [Non-patent Document 2] 3GPP2 C.S0024-A cdma2000 High Rate Packet Data Air Interface Specification Version 1.0, Chapter 11.7.6.1.5 and Chapter 14.2.1.3.3.3.